VoIP (Voice over Internet Protocol) service is getting more and more popular due to the low price and the possibility of providing various value-added services. Now IP technology can be used not only in the backbone of telecommunication networks, but also in the “last mile” to the end user. Although there are lots of VoIP-related products available in the market, such as hardware VoIP phones and VoIP client software, these products are only available to PC terminals.
However, it would be very convenient for end users if they can directly use a normal POTS phone to access the VoIP service. Such a demand has led to the emergence of the Phone Adapter products which enable a POTS phone to make IP phone calls. A Phone Adapter is equivalent to a mini signaling and media gateway, and can perform bi-directional signaling and media stream transformation. So a POTS phone plus a Phone Adapter can be regarded as an IP phone with the same function (semantically) of the POTS phone. Using such a solution, the POTS phone which is already used by the end user for access to a public switch telephone network (PSTN) can be reused, and the end user is more familiar with a POTS phone than a VoIP phone or VoIP client software. Moreover, if there are both POTS and VoIP services available, the user could use only a POTS phone to access the both services.
Since the function set of the POTS phone service is a subset of that of the VoIP service, the POTS user cannot use many advanced functions provided by the VoIP service, such as Call Transfer. The Call Transfer function is normally provided by a PBX in a PSTN, while for the VoIP service, it is only a client side function. Although the VoIP service comprises all the PSTN services, since the POTS phone does not support the advanced VoIP functions, the user is not able to use these functions via a POTS phone.